1. Field of the Invention
The present invention relates to a housing for retaining a gas generating composition for inflating an air bag and a method for manufacturing the same.
2. Description of the Related Art
An air bag system is installed in a sterring wheel of an automobile to prevent the driver from suffering possible injuries in an automobile accident. This air bag system generally comprises an air bag, a gas generating composition for inflating the air bag and a housing for retaining the gas generating composition. Since steering wheels equipped with an air bag system are heavier than steering wheels without air bag system, the steering manipulation for the former type is likely to be unstable. In this respect, it is desirable that individual parts constituting the air bag system, particularly, the housing for the gas generating composition be light in weight. To improve the specific fuel consumption to save the energy, it is desirable to make the bodies of vehicles as light as possible. In this respect, it is also desired that the components of the air bag system are made lighter.
Further, for the purpose of reducing injuries of drivers in accidents as much as possible, there is a demand of installing an air bag system in many vehicles. Fulfilling the demand requires that the mass productivity of individual components of each air bag system be increased so that the air bag system may be provided at cheapest possible prices.
With the above points in mind, a conventional housing for a gas generating composition as disclosed in, for example, U.S. Pat. No. 4,547,342, has a structure shown in FIG. 7.
Referring to FIG. 7, a ridge 32 has a top-open cylindrical shape with a bottom, and has a flange 32a. A housing 31 has a bottom-open cylindrical shape with a top. The housing 31, is fitted in the ridge 32, and is connected thereto by friction welding. The ridge 32 and housing 31 are made of aluminum. This housing is attached through the flange 32a to a base plate (not shown) of a steering wheel. An igniter chamber 33 is provided at the center of the housing in the space defined by the ridge 32 and the housing 31. A squib 34 is provided upright at the lower center portion in the igniter chamber 33. The igniter chamber 33 is filled around the squib 34 with an igniter material 36 for igniting a gas generating composition when heated by the squib 34.
Around the igniter chamber 33 is provided a combustion chamber 38, with an intermediate wall 37 in between. This combustion chamber 38 communicates with the igniter chamber 33 via a plurality of ports 39 formed through the intermediate wall 37. The combustion chamber 38 is filled with pelletized gas generating composition 35. Around the combustion chamber 38 is provided a cooling chamber 41, with an intermediate wall 40 in between. The cooling chamber 41 communicates with the combustion chamber 38 via ports 43 formed through the intermediate wall 40. In the cooling chamber 41 and along the peripheral portion inside the combustion chamber 38 is installed filtering material 44, which cools gas generated by combustion and decomposition of the gas generating composition 35 and at the same time filters other solid by-products than the generated gas.
In the case of the above housing for a gas generating composition, however, the ridge 32 and the housing 31, respectively, are integrally formed of aluminum that has substantially a poor material strength. To strengthen these two components; therefore, a thick walled material must be used, which impedes the air bag system from becoming lighter. In addition, those members 32 and 31 are formed by cold forging to ensure the desired strength, which stands in the way of improving the mass productivity and thus has an economical disadvantage.
There is another gas generating device with a structure different from the above type, as disclosed in Examined Japanese Patent Publication No. 62-5094. This gas generating device for a gas generating composition will be described referring to FIG. 8.
An igniter 51 is disposed in the center portion of the gas generating device. A squib 52 is disposed at the inner bottom portion of the igniter 51, with an igniter material 53 filling around the upper portion of the squib 52. Secured to the outer surface of the igniter 51 is the inner periphery of a ring-shaped casing 54, welded by electron beam irradiation. The casing 54 is formed by cold working.
The casing 54 constitutes a bottom wall 58 of a combustion chamber 57, and a top wall 59 and a peripheral wall 60 of a cooling and filtering chamber 55. A cover 61 for the combustion chamber 57 is securely welded between the igniter 51 and the peripheral wall 60 of the casing 54, which has an inverted-U shape in cross section. The combustion chamber 57 is filled with a gas generating composition 63. The combustion chamber 57 communicates with the interior of the igniter 51 via ports 64 bored through the peripheral wall 62.
A ring-shaped support plate 65 is fixed to the lower portion of the peripheral wall 60 of the casing 54. The support plate 65 and the peripheral wall 60 of the casing 54 define the cooling and filtering chamber 55, which is filled with a filtering material 66. This chamber 55 communicates with the combustion chamber 57 via a port 67 bored through the peripheral wall 60 of the casing 54. A gas exhaust port 56 is formed through the peripheral wall 60.
In the above-described housing for a gas generating composition, the casing 54 having a cross section extending zigzag in the longitudinal direction constitutes the bottom wall 58 of the combustion chamber 57, and the top wall 59 and peripheral wall 60 of the cooling and filtering chamber 55. This housing therefore has a higher mechanical strength than the one disclosed in the aforementioned U.S. Pat. No. 4,547,342, so that its casing 54 can be made with thinner walls. Actually, the casing 54 is designed with a wall thickness of about 2.5 mm. Further, since the casing 54 is fixed to the igniter 51 by electron beam welding, it can be formed lighter with reduced welded area. The combustion chamber 57 is however formed by welding the casing 54 to the cover 61. The cover 61 is provided on the side where an air bag 65 will extend, i.e., on the driver side. If the pressure in the combustion chamber 57 rises above the design limit of pressure resistance of the gas generating composition housing due to some reason, such as clogging of the filtering material or the housing being enveloped in flames, the cover 61 for the housing can be broken. This in turn may injured the driver.
Furthermore, the casing 54 has portions uneven in thickness. This makes it impossible to provide the housing simply by pressing a plate with a uniform thickness, and requires machining, such as grinding. This housing for a gas generating composition therefore is not suitable for mass production.